FIG. 1A illustrates a portion of a hydrocarbon well 100 extending through an interval of a subterranean formation 101. The well comprises a production tubing 103 and an annulus 102 around the production tubing. If the formation 101 contains unconsolidated particles, such as sand, produced fluids from the formation can carry those particles into the annulus 102, where the particles can enter the production tubing 103 and abrade or clog production equipment. The particles can also cause wellbore damage by clogging producing formations.
Well operators/service providers can place gravel packs within the wellbore to prevent formation particles from entering the wellbore and damaging the wellbore or equipment. Gravel packs comprise relatively coarse particulate material, such as graded sand, gravel or proppant. The gravel pack can be placed within the annulus of wellbore against the unconsolidated (or poorly consolidated) formation to form a barrier to formation particulate entering the wellbore.
FIG. 1A shows a gravel pack 104 placed against the formation 101 within the annulus 102 of the illustrated interval of the wellbore 100. The gravel pack 104 is held in place by a screen 105. The screen 105 is perforated with openings sized to exclude the gravel pack 104 material but to allow formation fluids to enter the annulus 102. The process of placing a gravel pack within an interval of a wellbore, known as “gravel pack completion,” is not relevant to this disclosure and will not be discussed.
The section of the wellbore 100 illustrated in FIG. 1A is referred to as an “open hole” wellbore, meaning that the wellbore does not contain a casing. FIG. 1B illustrates a section of cased wellbore 110. The cased wellbore 110 contains a casing 106 cemented in place by cement 107. The casing 106, cement 107, and a portion of the formation 101 have been perforated by perforations 108, as is known in the art. Following the perforation operation, a gravel pack 104 and screen 105 have been installed in the wellbore to prevent unconsolidated particles from entering the annulus 102.
Ideally, a gravel pack 104 would remain uniformly distributed throughout the life of the well. However, non-uniformities within the gravel pack may occur during gravel pack completion or may develop over time. Such non-uniformities can adversely impact the effectiveness of the gravel pack, especially if voids within the gravel pack allow formation fluids to enter the annulus.
Well operators evaluate the uniformity and porosity of their gravel packs using a process known as “gravel pack density logging.” Gravel pack density logging generally involves using a wireline to run a tool into the wellbore. Various types of density logging tools exist. Several types of density logging tools use a radiation source to irradiate the gravel pack and the surrounding formation (or to induce radiation within the gravel pack and/or formation, as described below). Such tools also include one or more detectors to detect radiation back-scattered from the gravel pack, formation, and/or materials in the annulus to evaluate the density of the gravel pack. Generally, when the gravel pack is less dense, a greater amount of radiation will be received at the detector. A denser (i.e., less porous) gravel pack captures more of the backscattered radiation, preventing it from reaching the detector.
Gravel pack density logging tools, as described above, can determine the relative density or porosity of a gravel pack, but do not provide quantitative measurements. In other words, the tool can indicate that one location is more or less dense than another location, but is incapable of providing an absolute density value for either location. Thus, there is a need in the art for gravel pack density logging tools and methods that can provide quantitative information about the density of gravel packs.